1. Field of the Invention
The present invention is an apparatus useful for measuring either the surface profile and the first derivative of the surface profile of a reflecting surface, or the light beam deviating property of a transparent article. It is especially suited for measuring large quantities of test articles quickly, automatically, and economically with non-contacting, non-destructive means.
2. The Prior Art
In many industrial operations it is necessary to measure the surface profile of a reflecting surface, or to measure some light beam deviating property of transparent articles of large quantities of semi-manufactured and finished products.
Previously known contacting techniques for measuring surface profile are not feasible for measuring large quantities of articles. They generally require contact between the test article surface and a reference surface or a probe, thereby imposing stringent requirements of care on the operation to prevent damage to the test article. Optical interference and Moire fringe are typical methods using a reference surface. Both methods yield a fringe pattern corresponding to contour lines of constant elevation. The departures of the fringe pattern from a reference pattern are indicative of the deviations of the test article surface geometry. An operator must visually evaluate the fringe pattern. If the surface deviations are irregular and correspond to more than one fringe, the resulting fringe pattern is so complex as to preclude easy interpretation and quantitative use.
Measuring machines which move either a contacting or non-contacting probe over the test article surface are generally quite slow because of the time required to trace a useful number of contours. Furthermore, the need for precise mechanical scanning is costly, and great care is needed to assure reliable operation.
Optical sensors have advantages because of the nature of light itself. The principal advantages are:
1. Optical measurements can be extremely accurate. PA1 2. A light beam can be scanned rapidly and precisely. PA1 3. Light variations are directly convertible to electrical signals. PA1 4. The response time is limited to that of the photodetector and its electronics. PA1 5. They do not require direct mechanical contact between the sensor and the object to be measured. PA1 6. Light can be used to carry out transmissive measurements. PA1 7. Light does not chemically deteriorate or deform the surface under test. PA1 8. The distance from the sensor to the object to be measured can be large. PA1 9. The measurements are independent of the chemical composition of the object.
Prior-art optical methods have not always proved satisfactory since many such devices are difficult to use, provide insufficient data, or data which are difficult to interpret.
Prior art non-contacting optical methods include interferometry, image blur detectors, and hybrid interference -- Moire apparatus.
Interferometers provide high sensitivity and accuracy. The interferometer output is a fringe pattern corresponding to a pattern of contour lines. The departures of the fringe pattern from a reference pattern are indicative of the light beam deviating properties and, hence, of the surface geometry of a reflective test article or the deviations of some light beam deviating properties of a transparent test article. An operator must visually evaluate the fringe pattern. If the deviations correspond to more than one fringe (typically .lambda./2 or 0.000012 in.) and are irregular, the resulting fringe pattern is so complex as to preclude easy interpretation and quantitative use. By photoelectrically sensing the phase information in the fringe pattern, it is of course possible to obviate the above problems, but at a substantial increase in complexity and cost.
Essentially, for many applications, an interferometer is too sensitive. An interferometer's sensitivity can be reduced by working at high angles of incidence to the test article surface. To achieve a reasonable decrease in sensitivity, the angle of incidence is very large, e.g., 80.degree. - 85.degree.. While the oblique incidence interferometry may be useful for some applications, problems with vignetting, the desire for variable sensitivity, and the need to measure transparent articles preclude its use for many others.
An apparatus for measuring automatically the flatness of mirrorreflecting surfaces is disclosed in Plummer et al. U.S. Pat. No. 3,761.179 issued Sept. 25, 1973. This technique is suited to measuring large quantities of articles and is essentially based on image blur sensing for its operation. It is essentially a photoelectric Hartmann test. Therefore, it works by sensing variations in light intensity produced by the nonflatness of the mirror surface under test. While this technique is useful for some applications, it is quite complex optically, mechanically, and electronically. Since it depends on variations of the light intensity to transduce the surface deviations, the outputs are not easily related to the surface topography.
Moire fringe techniques are also used to obtain contours of surfaces. Although the Moire fringe techniques are less sensitive than optical interferometry, a range of dimensions exists between those two techniques where neither is very useful.
An apparatus and method of measuring surface irregularities using a hybrid Moire-interference technique is disclosed in Jaerisch et al. in U.S. Pat. No. 3,858,981 issued Jan. 7, 1975. Since the output of the apparatus is a fringe pattern, a human operator is required to extract and to quantify the output. In addition, the apparent surface irregularities manifest in the output fringe pattern are those with respect to some datum plane of the apparatus and not with respect to the best fitting least squares plane. Therefore, this technique is extremely cumbersome, time consuming and expensive since a human operator is an integral part of the apparatus.
It has been suggested (Harrison -- IBM Technical Disclosure Bulletin Vol. 13 No. 3 Aug. 1970 pages 789-790) that the scanning of a specular or semi-specular surface with a collimated laser beam, and measuring the displacement of the reflected beam with a photoelectric position sensor, could be used to measure the surface profile of the surface. This technique is well suited to the problem; but the method suggested by Harrison has certain difficulties. In order to get the desired results, it is necessary that the surface of the test article be perpendicular to the incident light beam, or the resultant profile measurement will include the tilt of the surface. The necessity for such an adjustment precludes rapid measurements, since the mechanical tilt changes from sample to sample. Moreover, Harrison uses his collimating lens off-axis, thereby introducing a source of error into his measurements. The most serious limitation of Harrison's apparatus is that it provides only a surface profile along one line of the test article surface.
While these prior-art techniques for measuring surface profiles are useful for some measurements, they cannot be used for accurate measurements required in many industrial operations. For example, in the electronics industry it is desirable to measure the surface profile and nonflatness over the entire surface of the silicon wafers used in the manufacture of integrated circuits. Similarly, in the electronics industry it is desirable to measure the surface profile and nonflatness over the entire surface of the glass photomasks used in the photolithography portion of the manufacturing process. In both of these examples, the nonflatness of the surface is in the dimensional range from 0.000005 to 0.0005 in., i.e. .lambda./5 to 25.lambda., where .lambda.=6328A, the surfaces are specular reflectors, and large quantities of articles must be measured very rapidly. Similar requirements exist in the optical industry where a great variety of reflective surfaces and transparent products, e.g. lenses, lens assemblies, windows and prisms must be measured in large quantities accurately, rapidly and economically. To this end, measurement apparatus is required for both reflective surfaces and transparent articles which rapidly yields a signal indicating whether the deviations over the entirety of the test articles are within or outside a predetermined tolerance range.
In our copending application Ser. No. 492,781, we have disclosed a surface profilometer which is capable of rapidly measuring the surface profile and nonflatness of many articles, independent of the tilt of the surface, with easily variable sensitivity and good accuracy along a scan line.
In working with the embodiments of the invention disclosed in our above-identified application, we have encountered some problems. It is extremely difficult to produce a multi-faceted scanner prism which reflects incident light in exactly the same plane; the slight differences which are obtained with commercial prisms of this type introduce a slight dither in the position of the measurement beam which introduces a dither in the output of the instrument -- enough so that for some very precise measurements, the accuracy requirements are difficult to meet. More importantly, however, is the fact that when multiple scans over a single surface are desired, to essentially map a surface, it is difficult to get the desired accuracy. Movement of either the beam or the test object at commercially desirable measurement rates is either accompanied by a varying tilt between the scanning beam and the test object surface or is involved in expensive and intricate mechanisms for producing the desired movement in one plane while maintaining a fixed transverse plane.